Lower back pain is one of the most common, and one of the most expensive, disorders afflicting industrialized societies. Conservative treatments include rest, application of ice or heat, exercise, physical therapy, narcotics, steroids and weight control. If these treatments are insufficient to control pain and allow return to normal activity, surgical treatment may be required in which all or part of one or more degenerated, ruptured or otherwise failing discs is removed. This is followed by insertion of an interbody device, for example an artificial disc or fusion implant, and/or fusion of adjacent vertebrae. While fusion surgery is effective in a majority of cases, it has several disadvantages including a reduced range of spinal motion and an increased load transfer to adjacent levels of the spine, which accelerates degeneration at those levels and increases the likelihood of later problems with adjacent spinal segments. External stabilization of spinal segments, either alone or in combination with lumbar fusion and/or implantation of interbody devices, provides significant advantages over lumbar fusion alone, including prevention or reduction of pain.
U.S. Pat. No. 6,530,929 describes instruments for use in placing a brace, or stabilization device, in for example the spine. The brace comprises at least two anchors, such as pedicle screws, that are placed in adjacent vertebrae and a generally rigid rod that extends between, and is held in place by, the two pedicle screws. This spinal stabilization system, known as the Sextant™ system, is commercially available from Medtronic, Inc. (Minneapolis, Minn.). The Sextant™ system, which employs multiaxial pedicle screw implants and pre-contoured rods that are inserted percutaneously, requires the surgeon to make three, relatively small, incisions in order to place two pedicle screws and the rod—one to insert each of the screws and one to insert the rod.
A similar system, known as the Viper™ System, is available from DePuySpine, Inc. (Raynham, Mass.). While the Viper™ system also employs pedicle screws and a generally rigid rod, it only requires the surgeon to make two incisions—one to insert each screw. This is achieved by introducing the rod through a closed screw extension using a rod holder that is rotated through 90°. While systems such as the Viper™ and the Sextant™ systems can be employed to stabilize the spine, they have the disadvantage of preventing any motion between the two adjacent vertebrae.
In order to overcome this problem, dynamic stabilization systems have been designed that are intended to stabilize the spine by controlling abnormal spinal motion while preserving near normal spine function. US Patent Publication No. 2005/0143737 describes a stabilization system that employs at least one flexible element interposed between a bone anchor, such as a pedicle screw, and a generally rigid stabilization member, such as a rod or plate. Such a system, which would be difficult to implement in practice, is not yet commercially available.
The Dynesys™ system from Zimmer, Inc. (Warsaw, Ind.) is a dynamic stabilization system that is designed to bring lumbar vertebrae back into a more natural anatomical position while stabilizing affected spinal segments. This system, which is described for example in U.S. Pat. No. 7,073,415 and European Patent EP0669109B1, the disclosures of which are hereby incorporated by reference, is designed to be used either as a stand-alone treatment or in conjunction with fusion surgery. The system includes at least two pedicle screws that are anchored in adjacent vertebrae, and a flexible stabilizing cord that is threaded through, and extends between, the pedicle screws. The stabilizing cord consists of functional, working and inserting zones having varying thickness and flexibility. A separating cushion, or spacer, through which the cord passes, is positioned between the two pedicle screws. The stabilizing cord limits bending movements while the spacer holds the spinal segments in an anatomically functional position.
The Dynesys™ system is implanted by exposing the back of the spinal segment, inserting the pedicle screws into the vertebrae, cutting the spacers to the correct size, and putting the stabilizing cord in place. When employed to stabilize more than one spinal segment, the spacers are inserted segment by segment. The stabilizing cord is fixed in the eyes of the pedicle screws by mean of set screws. The surgeon can pretension the stabilizing cord separately for each spinal segment before fixing the cord in the pedicle screws, using a specially designed instrument. The stabilizing cord is then cut to the required length and the wound is closed. The main disadvantage of the Dynesys™ system is that significant spinal exposure and paraspinous muscle stripping is necessary in order to place the hardware, requiring the surgeon to make a relatively large incision. This leads to increased trauma with an associated increase in recovery time and risk of complications. In addition, the instrumentation for the Dynesys™ system is clumsy and does not permit a percutaneous approach.
US Patent Publication No. 2005/0065516 discloses a spinal fixation device comprising two securing members, such as pedicle screws, and a flexible metal connection unit connected to the two securing members, wherein the metal connection unit comprises a metal tube or pipe. In certain embodiments, the outer surface of the metal tube is provided with spiral cuts or grooves to provide a desired level of flexibility.
Many commercially available systems for spinal stabilization employ polyaxial screws, as described, for example, in U.S. Pat. No. 5,891,145. Polyaxial screws comprise a bone engaging portion, or fastener, and a head portion that may be positioned at any of a continuous range of angles relative to the bone engaging portion, thereby allowing a surgeon some flexibility in positioning the screw in the vertebrae. However, polyaxial screws are limited in terms of the amount of force that can be applied to them compared to fixed-head screws, as slippage between the head portion and the bone engaging portion tends to occur at forces greater than 200-300 Newtons.
There remains a need for an effective dynamic spinal stabilization system that can be implanted in a patient using minimally invasive procedures.